1. Field of the Invention
The present invention relates to a method for fixing a piercing nut.
2. Discussion of the Background
The conventional piercing nut is structured such that the pilot portion punches out the metal plate so as to pass therethrough, and caulks a peripheral edge portion of the punched through hole so as to be fixed to the metal plate.
For example, a rectangular type piercing nut (commonly called a universal type or a flat type) which has been known by U.S. Pat. No. 2,707,322, U.S. Pat. No. 3,152,628 and the like is the most basic structure, however, on the grounds that an embossing (see FIG. 5 of U.S. Pat. No. 2,707,322) is necessary in the metal plate portion of the nut punching portion, in order to prevent the pilot portion which is large scaled and punches out the metal plate so as to pass through from protruding from a lower surface of the metal plate so as to work against a thread fastening work, it has been hardly used at present.
On the other hand, there has been known, for example, from Japanese Examined Patent Publication No. 8-29392, Japanese Patent No. 2816645, Japanese Patent No. 3124357 and the like, a piercing nut in which a side wall protruding in such a manner as to surround the pilot portion is provided along an outer peripheral edge of a nut main body, an annular groove is formed between the pilot portion and the side wall, and the nut main body is formed into a quadrangular shape or a hexagonal shape and an outline shape of the annular groove is formed into a noncircular shape in order to improve a rotational drag (a force preventing a rotating motion of the piercing nut), as a high-stress type (or concave type) piercing nut.
In the high-stress type (or concave type) piercing nut mentioned above, since a height of the pilot portion punching out the metal plate so as to pass through is larger than a thickness of the metal plate, and the annular groove is provided, a whole of the piercing nut becomes large in scale. Further, it is necessary to provide a caulking projection in an upper surface of a punching die for pressure inserting a punched peripheral edge portion of the metal plate punched by the pilot portion to the annular groove, and there is a problem that a frequency of fracture is extremely increased in the case that the metal plate becomes thicker, in the caulking projection.